As the endogenous ligands for human GPR7 and human GPR8 (Genomics, 28, 84-91, 1995, WO 95/12670), which are G protein-coupled receptors, the ligands for GPR8 (sometimes referred to as NPW or neuropeptide W in the specification) were isolated and identified (WO 01/98494, WO 02/44368, J. Biol. Chem., 277, 35826-35832, 2002). In addition, NPW was shown to be the ligand for rat or mouse GPR7 (the same receptor as TGR26 described in WO 02/44368) (WO 02/44368, J. Biol. Chem., 277, 35826-35832, 2002). As physiological actions of NPW, the action of increasing food intake, the action of stimulating prolactin release, etc. by intracerebroventricular administration are known (WO 01/98494, J. Biol. Chem., 277, 35826-35832, 2002).
Further studies about the physiological actions of NPW or the involvement of NPW in diseases are required, and an assay system for detecting and quantifying NPW in a simple way with high sensitivity has been earnestly desired.